1. Field of the Invention
The present invention relates to an imaging optical system and an imaging apparatus using the same.
2. Description of the Related Art
In recent years, along with a tendency to reduce the thickness of cellular phones, a camera module in which an optical system's length in the direction of an optical axis is minimized at its limit, is demanded. Further, along with the increases in size and the pixel number of recent imagers, a lens having high resolution power is demanded. To cope with these demands, fixed focal-length optical systems having five aspheric lenses have been proposed. See Japanese Patent Application Publications Nos. 2007-264180 and 2010-48996.
The optical systems described in the aforementioned Japanese Patent Application Publications have high optical performances and the F-number (Fno) of each is rather small. However, these optical systems have a long whole length and a large maximum effective lens diameter. Therefore, when a designer tries to modify such a system to reduce its F-number and size, the effects of several aberrations, such as coma aberration, spherical aberration, field curvature and chromatic aberration, become large. Consequently, realizing compact optical systems with well-corrected aberrations has been difficult.